Sound is produced by the vibration of an object, which has three basic physical parameters, namely frequency, amplitude and phase (Figure 1). The unit of frequency is Hertz (Hz), and 1 Hz means 1 vibration back and forth in 1 second, and the human ear can perceive sound at 20-20 000 Hz. Amplitude represents the intensity of a sound and is measured in physical and hierarchical measures. The physical measures are sound intensity and sound pressure. Sound intensity is the energy of a sound wave acting per unit of time on a unit area perpendicular to its direction of transmission, and sound pressure is the difference in pressure between the center of the transmission medium (e.g., air, water, solids, etc.) with and without sound wave propagation. Since the human ear can hear a large range of sound intensity, the difference between the maximum and minimum values 1012 times, and the human ear feels the size of the sound proportional to the logarithm of the ratio of the two sound intensities. Therefore, it is more convenient to use the logarithmic scale to express the level of sound intensity in Bells (B) or decibels (dB). Phase is the position of a vibrating mass in the vibration cycle at a given moment, and is the scale of whether the mass is at the crest of a wave, the trough of a wave, or some point between them. It is usually measured in degrees (angles) and is also called the phase angle. When the vibration waveform varies in a periodic manner, the waveform cycle is 360o for one week, and the amplitude of the masses in each phase is different. The vibration transmitted by the sound source reaches the left and right ears in different phases, thus producing different stimuli to the left and right ears respectively, and we thus locate the sound source. Figure 1. The three parameters of sound waves. Figure 2. Schematic diagram of the human auditory system. The auditory system consists of the outer ear, middle ear, inner ear, and auditory center (Figure 2). The outer ear includes the auricle, external auditory canal and tympanic membrane. The shape of the auricle facilitates the aggregation of sound energy, the collection of sound and the determination of the location of the sound source. The external auditory canal is a channel for sound transmission, with one end opening in the center of the auricle and one end terminating in the tympanic membrane, and is about 25 mm long. It is also an effective resonance cavity, allowing weaker sound waves to be reinforced and causing the tympanic membrane to vibrate. The resonant frequency of the human external auditory canal is 3kHz-4kHz. due to the difference in the properties of air and liquid, direct transmission of sound waves from the air to the inner ear lymphatic fluid will result in an energy loss of 30-36dB. there are three small bones in the middle ear (including the hammer bone, anvil bone and stapes) that act as levers, which increase pressure by 1.3 times, and the area of the tympanic membrane is 18.6 times the area of the stapes floor vibrating the inner ear lymphatic fluid, both of which increase pressure by 27.6dB. The total pressure increase is 27.6 dB, which basically eliminates the energy loss caused by the direct transmission of sound waves from the air to the inner ear lymphatic fluid. The inner ear includes the cochlea and vestibule, and the Corti’s apparatus in the cochlea implements the acoustic-electrical conversion process. The vibration of the lymphatic fluid stimulates the hair cells of the Corti’s apparatus, converting the mechanical vibration energy into a changing electrical signal, which is transmitted to the auditory center through the nerve cells and their fibers (Figure 3). The auditory center (temporal lobe of the brain) precisely analyzes the electrical signals and assigns various meanings to them. Figure 3. Auditory conduction pathway from the cochlea to the center.